page_title: Shortt–Synchronome clock
section_title: Reason for accuracy
hierarchical_section_title: Shortt–Synchronome clock / Reason for accuracy
caption_attribution_description: English: Copper low pressure tank containing the master pendulum of a Shortt Synchronome astronomical regulator clock, the most accurate pendulum clock ever manufactured, in the World Museum, Liverpool. The pendulum was electrically linked to a slave pendulum in a separate clock unit standing several feet away (out of shot). The slave pendulum connected to the clock gears, leaving the master pendulum to swing free of disturbance. This clock was the highest standard of timekeeping, used as a primary time standard from the 1920s until atomic clocks became available in the 1970s. It had an accuracy of better than a second per year, sufficiently precise to measure variations in the rotational speed of the Earth. Also used in WWII to calibrate ASDIC systems. Invented by William Hamilton Shortt and made by the Synchronome Company in 1937.
context_page_description: The Shortt–Synchronome free pendulum clock was a complex precision electromechanical pendulum clock invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones, and manufactured by the Synchronome Co., Ltd. of London, UK. They were the most accurate pendulum clocks ever commercially produced, and became the highest standard for timekeeping between the 1920s and the 1940s, after which mechanical clocks were superseded by quartz time standards. They were used worldwide in astronomical observatories, naval observatories, in scientific research, and as a primary standard for national time dissemination services. The Shortt was the first clock to be a more accurate timekeeper than the Earth itself; it was used in 1926 to detect tiny seasonal changes in the Earth's rotation rate. Shortt clocks achieved accuracy of around a second per year, although a recent measurement indicated they were even more accurate. About 100 were produced between 1922 and 1956.
context_section_description: A pendulum swinging in a vacuum without friction, at a constant amplitude free of external disturbances, theoretically keeps perfect time.  However, pendulums in clocks have to be linked to the clock's mechanism, which disturbs their natural swing, and this was the main cause of error in precision clocks of the early 20th century. An ordinary clock's mechanism interacts with the pendulum each swing to perform two functions: first, the pendulum must activate some kind of linkage to record the passage of time.  Second, the clock's mechanism, triggered by the linkage, must give the pendulum a push (impulse) to replace the energy the pendulum loses to friction, to keep it swinging.  These two functions both disturb the pendulum's motion.
  The advantages of the Shortt clock are first, it reduced the disturbance of the master pendulum due to the impulse by only giving the pendulums an impulse once every 30 seconds exactly (30 pendulum swings), and second, it eliminated all other interaction with the master pendulum by generating the necessary precise timing signal to control the slave clock (and record the passage of time) from the impulse mechanism itself, leaving the pendulum to swing "free" of interference.
caption: Master pendulum tank
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